The upside is the possibilities contained in knowing that everything is up to us; where before we were experts in the array of limitations, now we become authorities of what is possible.

分号的作用可以相当于一个并列连词。所以，这是两个并列分句。

1.  The upside is the possibilities contained in knowing that everything is up to us; （分号）

2.  where before we were experts in the array of limitations, now we become authorities of what is possible.

第一并列分句：

The upside （主语）is（系动词） the possibilities（表语） contained in knowing that everything is up to us（表语的定语；由过去分词充当）

第二并列分句：

where before we were experts in the array of limitations（where 引导对比从句）, now we become authorities of what is possible.（主句）

where (用于引导从句进行对比) 然而，跟 while 用法相同：

You use where to introduce a clause that contrasts with the other parts of the sentence.
例如：

Where some people learned to play the accordion for dances in their community, others took music lessons.
一些人学习弹手风琴，为他们社区里的舞蹈表演进行伴奏，而其他人则上音乐课。
Sometimes a teacher will be listened to, where a parent might not.
有时教师的话会听，而父母的话可能就不听。

upside：好处，好的一面，优势。

【翻译】好的一面是，知道一切都由我们作主是有可能的；以前我们是有一系列局限的专家，然而现在我们已成为可能的权威。